The water is over buffered due to high alkalinity. This is a not uncommon problem in hard water areas where the mains feed water has a high total alkalinity.

Solutions:

Add dry acid to lower the total alkalinity to below 200ppm. The dose rate is 2kg per 100 cu metres (22,000 galls) of pool water. It is important to add the acid a little at a time and pre-dissolved at a dilution no stronger than 8:1.